[41.14] A NICMOS look into the mass function of galactic globular clusters

The NICMOS NIC3 camera on board HST observed serendipitous
stellar fields in several Galactic globular clusters during
the parallel campaign. In particular, deep exposures were
taken in the J and H bands of NGC 288 and NGC 7078, which
allowed us to derive the mass function in the cluster
outskirts. Observations of a field located 5'.4 SE of the
centre of NGC 288 and reaching limiting magnitudes J=25 and
H=24 have allowed us to trace, for the first time, the
luminosity function of NGC 288 over the range 3 < MH <
9 and peaking at MH = 6.8. We have used this information
to constrain the cluster mass function which is best fitted
by a log-normal distribution with characteristic mass of
0.42 Mo. This is fully consistent with the recent
analysis of the luminosity functions of all globular
clusters observed so far with HST showing a mass function at
the half-light radius that appears ``undistorted'' by
evaporation or tidal interactions with the Galaxy. This
scenario is further confirmed by the data collected with
NIC3 in three fields located 7' NE of the centre of NGC
7078, which reveal a luminosity function extending in the
range 2 < MH < 9 with a peak at MH = 6.89. The
corresponding mass function is best fitted with a log-normal
distribution with characteristic mass of 0.30 Mo. When
compared with WFPC2 data obtained 4'.6 NW of the cluster
centre, the mass distribution at 7' indicates that there
are no radial variations in the mass function of NGC 7078
outside the central ~ 20'' radius, where mass
segregation is dominant. These results strengthen the
recently proposed hypothesis that the present mass function
observed at the cluster half-light radius reflects very
closely the IMF, which might be generally represented by a
log-normal distribution peaked at ~ 0.3 Mo.